1. Field of the Invention
The present invention relates generally to reducing the amount of fluid normally maintained in the bottom of a tank and, specifically, reducing the amount of undrawable oil located below the top edge of the suction line in an oil storage tank.
2. Description of the Prior Art
In the modern world the storage of petroleum, both crude and refined, is an important part of efficient energy management. The typical oil storage tank, whether fixed or floating roof, has an inlet pipe and intake valve located in the side of the tank near the bottom and a corresponding suction line and delivery valve also located in the side near the bottom of the tank. The tank bottom is generally sloped downwardly toward the middle to collect water and other sediment, which settles out during storage. A valved water draw off line is connected to the lowermost point in the tank so that water and sediment that has collected can be periodically removed. The suction line or outlet is round and may be typically one to three feet in diameter. It is located with its bottom edge as close to the bottom of the tank as is consistent with ensuring that no water or sediment enters the suction line, usually about one foot is sufficient. When drawing off the stored oil, the oil level can not be permitted to fall below the top edge of the suction line, otherwise the pump will suck air and other vapors in the tank and lose its prime, thereby becoming vapor locked, and be ineffective. Therefore, there exists at the bottom of all oil storage tanks a layer of oil, of a depth at least equal to the diameter of the suction line, with reasonable margins above and below. This layer of oil is typically up to four feet deep and is not available for use during the period the tank is in service. This is the deadstock or undrawable bottoms. Only at the time that the tank is totally drained through the water drawoff line can this oil be recovered and made available for use. Such draining takes place only about once every five years when the tank is emptied and cleaned. As soon as the tank goes back into service, the deadstock is there again. Given the large size of most oil storage tanks and an approximate price of $30 a barrel, it can be easily seen that vast amounts of oil worth millions of dollars are presently represented by these "undrawable bottoms" or "heels" in the bottoms of the existing oil storage tanks. The present requirement to maintain a relatively large inventory due to uncertainties of availability and price is in no way assisted by this deadstock, since it is undrawable and, therefore, not available to meet product demands.